This invention relates generally to tissue paper machines, and more particularly, to a method for use in connection with tissue paper machines for dewatering a web prior to passing it to a flow through drying cylinder.
In the past in order to dewater a paper web so that it has a sufficiently high dry matter content, the web has been commonly subjected to a wet pressing operation, i.e., has been directed through press rollers located in front of the flow through dryer. However, this has not been entirely satisfactory in tissue paper making since such wet pressing reduces the desirable bulkiness, softness, and absorbing capacity required of tissue paper. In order to overcome this problem, various types of suction apparatus had been employed in conjunction with wire fabric transfer systems in order to ensure sufficient dry matter content in the web without using wet pressing. However, this method is also not entirely satisfactory for the reason that the use of suction energy for removing water from the web is unusually uneconomic.
Recently, flow through dryers have gained wide spread use in tissue paper machines. Such flow through dryers are conventional and generally comprise a drying cylinder having a perforated flow through mantel and a surrounding hood in which an overpressure is provided. The interior of the flow through drying cylinder is provided with vacuum apparatus such that hot drying gases are drawn from the hood through the web and cylinder mantel to the drying cylinder interior. As is known, the flow through drying apparatus may be designed to provide flow of hot gases from the cylinder to the hood.
Where flow through drying is used in tissue paper making machines, wet pressing of the web is usually avoided in order to maintain the softness, bulkiness and absorbing capacity required of tissue paper.
Thus, tissue paper machines are known which include large diameter flow through drying cylinders and subsequent creping cylinders from which the web is removed by creping scrapers in a known fashion. Tissue paper machines are also known which include conventional yankee cylinders and subsequent flow through cylinders which act as after dryers. For example, reference is made to the article "Fortschritte in der Durchstromtrockungstechnik" in "Das Papier", Number 10a 1976, pages 118 to 127.
According to previously known methods, a web is transferred from the former wire to the flow through drying cylinder by wires which function not only to transport the web from the former section to the drying cylinder but which also function as a flow through drying wire which conducts the web over one or more flow through drying cylinders. However, it has been found that in the use of such wires as transfer fabrics, a problem has arisen in that washing water often splashes over from the wire onto the web. Additionally, it has been found that water droplets which fall on the wire fabric may result in deleterious stains in the paper.
Where such wires are used in transporting the web from the former section to the flow through drying cylinder, it is necessary to provide a relatively high suction through the wire in order to ensure adequate adherence of the web against the wire.
In such cases, water is drawn from the web through the wire where it is removed by the suction air current. Where the magnitude of the suction is decreased, it is then necessary to provide the lower suction forces over a proportionately greater distance in order to ensure a sufficiently long relation of the suction so that the water is drawn from the web through the wire. In both cases, namely, high suction over a short distance or lower suction over a proportionately larger distance, large quantities of air are required.
Reference is made to U.S. Pat. Nos. 3,303,576 and 3,821,068 which relate generally to the present subject matter. Reference is also made to copending U.S. application Ser. No. 922,274 filed simultaneously herewith and assigned to the same assignee as the present application, which discloses subject matter generally related to the present subject matter.